As a high-speed and large-capacity network has been developed in recent years, an optical network that performs WDM (Wavelength Division Multiplexing) using an OTN (Optical Transport Network: ITU-T G. 709) has been put into practical use.
The OTN is a technique for accommodating client signals conforming to SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy), Ethernet, or the like and for performing data transmission suitable for the WDM by generating a frame having a hierarchal structure.
Further, when a failure occurs, a protection method for recovering communication at high speed is required. In a ring network, protection methods such as BLSR (Bidirectional Line Switched Ring: Telcordia GR-1230-CORE) and MS SPRING (Multiplex Section Shared Protection Ring: ITU-T G.841) have been used in general. In these protection methods, traffic is looped back at each of end nodes of a failed link along the ring so that the failure is bypassed.
A technique of recovering a failure of a WDN ring network is disclosed, for example, by Japanese Laid-open Patent Publication Nos. 2000-209244 and 2000-209152.